1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine provided with a catalyst capable of reducing nitrogen oxides (hereinafter, NOx) under oxidizing conditions and in the presence of hydrocarbons. More particularly, the present invention relates to an exhaust gas purification system wherein hydrocarbons having low boiling points produced from a fuel for the engine is supplied to the catalyst to increase a NOx purification rate of the catalyst.
2. Description of the Prior Art
Combustion at lean air-fuel ratios is effective to improve a mileage characteristic of automobile internal combustion engines, and such lean air-fuel combustion (lean burn) is actually executed in diesel engines and some types of gasoline engines. However, in the lean burn engine, NOx reduction by a three-way catalyst cannot be expected, and therefore, an alternative means for reducing NOx needs to be developed.
As a catalyst capable of reducing NOx even under an oxidizing exhaust gas condition of the lean burn engines, Japanese Patent Publication HEI 1-130735 discloses a zeolite catalyst carrying transition metals which can reduce NOx in the presence of hydrocarbons. Also, Japanese Patent Publication SHO 63-283727 proposes an apparatus for supplying hydrocarbons to an engine exhaust conduit upstream of the zeolite type catalyst. However, the apparatus includes a particular hydrocarbon source different from the fuel for the engine, and installation of such particular hydrocarbon source would increase cost.
To avoid installing such particular hydrocarbon source, it could be conceived to utilize a portion of the fuel for the engine to supply hydrocarbons to the zeolite type catalyst. However, it was found by the inventors through various tests that merely using fuel will not improve the NOx reduction rate of the zeolite type catalyst. More particularly, the hydrocarbons effective to reduce NOx have a relatively small carbon number (for example, 3-6) per molecule. In contrast, diesel fuel (light oil) mainly includes hydrocarbons having a large carbon number (for example, more than 10) per molecule. As a result, even if the fuel is supplied to the zeolite type catalyst, significant increase of NOx purification rate cannot be expected.